Sub-elite female swimmers (N = 10) were assessed for thyroid stress every other week during a 12-week training period. The group was divided into an improved (N = 5) or non-improved (N = 5) group based on end of season performances.

Free T3 and Free T4 were significantly lower in the non-improved group than in the improved group at all times across the observation period. Poor performing female swimmers showed signs of low T3 syndrome (abnormalities in thyroid hormone and thyroid-stimulating hormone levels), which might reflect an altered metabolic state.